With 14 months to go before Election Day, the House battleground continues to take shape. Even though there is some uncertainty about what the political climate will look like next fall and whether normal historical midterm trends will hold under President Donald Trump, the House playing field is expanding, almost entirely in the Democrats’ direction.

As we’ve mentioned plenty of times before (and will likely repeat over and over again), history puts the Republican Party at a disadvantage: The president’s party has lost seats in 18 of the last 20 midterm elections, with an average loss of 33 seats. Democrats need to gain 24 seats next year for a majority.

Midterm elections often a referendum on the president, and when voters disapprove of his performance, they punish his party because his name isn’t on the ballot. Historical trends are based on that dynamic.

But what happens when voters perceive the president to be outside the traditional two-party system? Trump is technically a Republican because he ascended through the GOP nominating process. Still, many voters see him as his own brand rather than as a party leader. If that differentiation continues, GOP candidates could avoid the typical midterm disaster.

It’s certainly possible that historical norms will remain in tact and voters will couple Republicans in Congress with the president. Plus, voters could become angered by members’ own voting records, or Trump might blame Republicans in Congress for the failures of the country. Any combination of those factors could be problematic for the GOP.

For now, we’ve changed the Inside Elections ratings in 15 House races, all but one of them to a more favorable category for Democrats.

Trump’s job approval rating continues to hover in mediocrity (39 percent of voters approved while 56 percent disapproved in the latest Real Clear Politics average), creating an uncertainty that is causing more GOP members to be potentially vulnerable. Our ratings (and these ratings changes) are the result of developments at the national and district level.

Race moving toward Republicans

Overall, the House playing field includes 48 seats currently held by Republicans and 14 seats held by Democrats. For some perspective, the House battleground is nearly twice as large as it was at the same point two years ago. In September 2015, the list of competitive seats included 25 Republican-held districts and just seven seats held by Democrats.

The current battleground is still probably too small for Democrats to win the majority. They would need to hold all of their own seats, win the two Republican seats they are already favored to win, all of the toss-ups, all of the Tilt Republican seats, and almost all of the Lean Republican seats. A Democratic majority is possible, but still not likely at this point.

Open seats, including retirements, are critical in shaping the House battleground.

But Democrats need more Republican retirements in competitive districts to decrease the number of well-funded and established incumbents they must defeat.

History tells us that there will be at least a dozen more House retirements, but the geography and partisanship of the open seats will be important in handicapping their impact on the fight for the majority.